A photosensitive resin composition containing a compound having a quinodiazido group such as a naphthoquinonediazido group, a benzoquinonediazido group, etc. has found use as a positive type photoresist, because, upon exposure to light having a wavelength of 300 to 500 .mu.m, the quinonediazido group decomposes to form a carboxyl group whereby the composition goes from alkali-insoluble to alkali-soluble. Such a resin composition is usually combined with a novolak resin which gives a uniform, tough resist film. The positive type photoresist is incomparably superior in resolving power to a negative type photoresist. Because of this high resolving power, the positive type photoresist is used as a protective film for photographic etching which is used for producing copper-clad printed-circuit boards and integrated circuits such as IC and LSI.
With the evolution of the integrated circuit, the degree of integration has reached such a level that patterns as thin as 1 .mu.m are required. According to the conventional processes for the production of integrated circuits, light exposure has been acomplished by placing a mask in intimate contact with the substrate. It is said that this process cannot make patterns thinner than 2 .mu.m. Instead of the conventional processes, the reduction projection exposure system has come to draw attention.
According to this new system, the pattern of a master mask (reticle) is projected with reduction by a lens system, whereby exposure is accomplished. This system realizes a resolving power of about 1 .mu.m. With the improvement of this system, a high-performance photoresist has come to be required. One of the important items of performance is a .gamma.-value. Assuming that when the exposure energy changes from E.sub.1 to E.sub.2, the rate of development in the exposed area changes from R.sub.1 and R.sub.2 (provided that the exposure energies E.sub.1 and E.sub.2 and the rates of developments R.sub.1 and R.sub.2 are taken within the range where a logarithm of the rate of development is in proportion to a logarithm of the exposure energy), the .gamma.-value will be a value as defined by the following equation: EQU .gamma.=(log R.sub.2 - log R.sub.1)/(log E.sub.2 - log E.sub.1)
The .gamma.-value is an index representing resolving power, and the greater the .gamma.-value, the greater the rate of development changes in response to the change of the exposure energy. As the result, the pattern becomes sharper.
Hitherto, a few efforts have been made to improve the -value and pattern size reproducibility of positive type photo-resist compositions through the modification of quinonediazide. However, the results were not satisfactory to meet the recent requirements for those performance. Thus the present inventors paid their attention to improving those performances through the modification of novolak resin which has so far been reglected.
Novolak resin has long been known as a general-purpose molding resin, and a large variety of attempts have been made for its improvement. However, molding-grade novolak resin cannot be used as such for positive type photoresists. For example, novolak resin for molding has a weight average molecular weight (Mw) lower than 2000, whereas a novolak resin for photoresist should have a molecular weight in excess of 3000. In addition, their raw materials such as phenol should be selected according to different standards.
The production of molding-grade novolak resin is disclosed to West Germany Pat. No. 1,022,005. According to this patent, novolak resin is produced by using as a catalyst an organic acid salt of a divalent metal having a greater ionization tendency than hydrogen. The novolak resin thus produced contains more ortho-ortho linkages than that produced by using an acid catalyst. Such a novolak resin is called high-ortho novolak resin. It has an advantage of curing faster than an acid-catalyzed resin when molded in combination with an amine hardener. The curing rate is not necessarily an important property for positive type photoresists. Moreover, the above-cited patent neither describes nor suggests anything about positive type photoresists.
With the foregoing in mind, the present inventors carried out a series of researches on the improvement of novolak resin. As the result, it was unexpectedly found that the positive type photoresist is greatly improved in .gamma.-value and pattern size reproducibility when it is produced from a specific novolak resin and a o-quinonediazide compound, the novolak resin being synthesized from a specially selected phenol by using as a catalyst an organic acid salt of a metal which is more electropositive than hydrogen. The present invention was completed on the basis of this finding.